JPH072971B2 - Method for manufacturing ferritic stainless steel sheet with good workability - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a ferritic stainless steel sheet having good workability.

(Prior Art) As described in JP-A-55-97430, for example, a stainless steel thin steel sheet is hot-finished after a slab continuously cast with a thickness of about 200 mm is roughly rolled or heated to 1200 ° C. It is rolled into a hot-rolled sheet, subjected to hot-rolled sheet annealing in a bell-type annealing furnace, cold-rolled and finish-annealed to obtain a product.

(Problems to be Solved by the Invention) As described above, the known technique has a large slab thickness and requires various energies for forming a predetermined hot-rolled sheet.

(Means for Solving Problems) In order to solve the above problems, the present invention aims to omit the hot rolling step and the hot-rolled sheet annealing step all at once, and the gist thereof is as follows. However, it is as follows.

(1) In wt%, Cr: 8-30%, C: 0.001-0.5% containing ferritic stainless steel is continuously cast to obtain a cast piece having a thickness of 3 mm or less, and the cast piece is solidified from the solidification temperature to 1200 After cooling to ℃ at a cooling rate of air cooling or more, then from 1200 ℃ to 1000 ℃ at a cooling rate of 30 ℃ / sec or more, then 1000 ℃ or less
A method for producing a ferritic stainless steel sheet having good workability, which comprises winding a thin cast coil in a temperature range of 700 ° C or higher, and then cold rolling and annealing to obtain a thin steel sheet.

(2) By weight, Cr: 8 to 30%, C: 0.001 to 0.5% containing ferritic stainless steel is continuously cast to obtain a slab having a thickness of 3 mm or less, and the slab is solidified from the solidification temperature to 1200. Cool down to ℃ at a cooling rate of air cooling or higher, then from 1200 ℃ to 1000 ℃
Cooling at a cooling rate of 30 ° C / sec or more, and the above 12
After the cooling process from 00 ℃ to 1000 ℃ or after finishing this cooling process, rolling process with a rolling reduction of 10% or more, and then winding in a temperature range of 1000 ° C or less and 700 ° C or more to obtain a thin cast coil A method for producing a ferritic stainless steel sheet having good workability, which comprises subjecting the sheet to cold rolling and annealing to obtain a thin steel sheet.

The present invention will be described in detail below.

First, in the present invention, the reason for limiting the slab thickness to 3 mm or less and cooling from the solidification temperature to 1200 ° C. at a cooling rate of air cooling or higher is to improve the ridging property.

Ridging is a type of surface defect that occurs when a thin steel sheet is processed, and is not observed in ordinary ordinary steel thin sheets, and is a phenomenon unique to ferritic stainless steel sheets. The reason for this is
It is considered that this is because the ferritic stainless steel does not undergo complete transformation and is less likely to be recrystallized as compared with ordinary steel, so that the structure is not easily refined during the manufacturing process. In other words, since it is difficult to reduce the grain size, there is a group of crystal grains with a specific crystal orientation (hereinafter referred to as colonies) on the product plate, and when processed, processing anisotropy occurs in colony units and observing as ridging To be done. Therefore, if the particles are made finer before the step of forming colonies during manufacturing, the colonies are also made finer and the ridging property is finally improved. For example, the conventional hot-rolled sheet annealing step is intended to recrystallize the structure of the hot-rolled sheet to make it fine grain before cold rolling. Furthermore, the present inventors proceeded with research based on the above idea, confirmed that the finishing hot rolling step was also a colony forming step in the production process of the α-stainless steel sheet, and recrystallized in the rough hot rolling step before that. Therefore, it has been found that the hot-rolled sheet annealing step can be omitted.
(Japanese Patent Application Laid-Open No. 59-13026, Japanese Patent Application No. 60-3002) The present invention further develops the above idea and omits the hot rolling step.

Generally, when the thickness of the cast piece is reduced, the solidification cooling rate and the cooling rate of the cast piece after solidification are increased, and the cast structure is refined. Fig. 1 shows the relationship between the slab thickness and the width of the columnar crystals in the cast structure when casting ferritic stainless steel containing 16% Cr. An example of the cast structure is shown in Fig. 2 and Fig. 3. Show.

The slab thickness of ferritic stainless steel is 1 from the conventional 10 ² mm.
^{0 1} mm, 10 0 mm ... and if we thin, the cast structure is substantially the size of the columnar crystals in the columnar crystal structure becomes smaller. Although the columnar crystals have hitherto been considered to be a factor for greatly deteriorating the ridging characteristic, they do not become a factor for deteriorating if the size thereof is reduced. The relationship between the ridging characteristics and the thickness of the cast piece is shown in FIG. From this figure, it is recognized that the ridging characteristics are improved by reducing the thickness of the slab. The reason for this is considered to be that by reducing the thickness of the slab, the structure can be made finer before the colony forming step (the cold rolling step in the method of the present invention), so that the colony size becomes smaller and the ridging characteristics are improved. The reason why the structure becomes finer when the thickness of the slab is reduced is that the solidification cooling rate and the cooling rate of the slab after solidification increase. That is, when the solidification cooling rate is high, the degree of supercooling of the liquid phase is high, and the number of solid phase nuclei is increased accordingly and the particles are finely divided. Further, since grain growth occurs due to grain growth even after completion of solidification, grain refining cannot be achieved unless the slab cooling rate is increased. With a normal thin wall slab continuous casting machine (single roll, twin roll, single belt, twin belt casting machine, etc.), the slab thickness is 3 mm.
By setting the following, the cooling rate during solidification can be increased to the extent that the desired target is achieved, but grain growth after solidification cannot always be suppressed, usually from the solidification temperature to the actual grain growth stop of ferritic stainless steel. It is necessary to cool to a temperature of 1200 ° C. at a cooling rate of air cooling or more, preferably 30 ° C./sec or more.

For the above reasons, the upper limit of the thickness of the cast piece is set to 3 mm or less, and the lower limit of the cooling temperature from the solidification temperature to 1200 ° C. is limited to the air cooling or more.

Although the lower limit of the thickness of the cast piece is not particularly limited, it is preferably 0.5 mm or more in order to stably produce the cast piece having a good shape. The upper limit of the cooling rate from the solidification temperature to 1200 ° C is also not particularly limited, but it can be controlled practically up to about 200 ° C / sec.

With respect to ridging, the refining effect of the cast structure has been described, but when manufacturing a thin steel sheet having a sheet thickness in which the hot rolling step is omitted as in the present invention, the slab thickness is reduced to reduce the cold rolling reduction rate. It is considered that there is also an effect of improving the ridging characteristics by reducing the size.

Next, the reason for cooling from 1200 ° C to 1000 ° C at a cooling rate of 30 ° C / sec or more and winding the slab in the temperature range of 1000 ° C or less and 700 ° C or more will be described. The reason for this is to improve the cold rolling property of the slab and the mechanical properties and deep drawing properties of the product plate.

As mentioned above, ferritic stainless steel does not undergo complete transformation, but it generally undergoes partial transformation in the high temperature range of 1200 ° C to 1000 ° C.
There are two phases, a phase and a γ phase. In the hot-rolled sheet in the conventional process, the low temperature transformation phase (α
(Harder than the parent phase), if cold-rolled in this state, cold rolling breaks, the reduction ratio during cold rolling does not become constant, and the thickness variation in the coil longitudinal direction becomes large. Cold rollability deteriorated, such as cracks in the edges.

Further referring to the material, the existence of this low temperature transformation phase (hard phase) inhibits the development of cold rolled texture,
Although the ridging property of the product sheet was improved, the deep drawing property was remarkably deteriorated, and the low temperature transformation phase was decomposed during the final annealing, which caused problems such as increasing the yield stress and decreasing the total elongation value. Therefore, in order to decompose the low temperature transformation phase in the hot rolled sheet into the α phase and the spherical carbide, the hot rolled sheet annealing step has been conventionally required. In order to omit this hot-rolled sheet annealing step, for example, a technique of ignoring deterioration of cold rolling property, deep drawing property, etc. at low temperature winding during hot rolling (Japanese Patent Publication No. 49-17932), deterioration of ridging property Technology for ignoring high-temperature winding during hot rolling
58-32217), γ → α during hot rolling and winding by adding Al
Technology for accelerating transformation and improving cold rolling property and material (JP-A-61)
Further, there is a technique (Japanese Patent Application No. 60-3254) for improving the material such as the deep drawing property and the cold rolling property by performing recrystallization at the time of rough hot rolling and then winding at high temperature during ridging.

However, all of the above techniques are premised on the hot rolling process, and regarding the cold rolling property, the γ phase is intended to be decomposed by high temperature winding during hot rolling. When the γ phase is decomposed by this high temperature winding, the carbide is difficult to be spheroidized, and there is a problem that the cold ductility is slightly inferior to the conventional annealed steel sheet.

Therefore, as a result of researching a method of solving the above-mentioned problems and omitting the hot rolling step and the hot rolled sheet annealing step at a stroke, the present inventors did not produce a large amount of the γ phase which causes the low temperature transformation phase. Similarly, by rapidly cooling the γ phase precipitation temperature range after solidification and gradually cooling in the two-phase region of α phase and carbide, supersaturated solid solution C is made into fine spherical carbides, cold ductility, deep drawing characteristics, etc. The method of the present invention for improving the quality of the material has been completed. That is, a slab of ferritic stainless steel with a slab thickness of 3 mm or less is produced at 1200 ° C to 1000 ° C.
Within the temperature range up to ℃, 30 ℃ /
After cooling at a cooling rate of at least 50 seconds / second, preferably at a cooling rate of at least 50 ° C / second, a thin cast piece is wound in the temperature range of 1000 ° C to 700 ° C to convert supersaturated C into a fine spherical carbide. is there. Needless to say, the effect is further enhanced if the thin cast coil wound up at this time is gradually cooled or heat-retained.

The thin slabs produced as described above are pearlite transformed from the γ phase (α phase and (F
Although the amount of e, Cr) ₂₃ C ₆ ) phase is small, the amount of finely precipitated carbide ((Fe, Cr) ₂₃ C ₆ ) in α grains is large, and the cold rolling property is good. Further, in the thin cast piece produced by the usual method, since the γ phase is precipitated in the grain boundaries of the α matrix (mainly random grain boundaries), carbides transformed from the γ phase (forming pearlite) are mainly Although it covers the grain boundaries of the α phase, the method of the present invention suppresses the precipitation of the γ phase and precipitates the carbides as described above, so that the carbides are finely dispersed in the α grains and the α grain boundaries are compared. It is clean. The α grain boundary is {11
It becomes a recrystallization nucleation site for 1} grains, and if carbides cover the α grain boundaries during cold rolling, it is less likely to become nuclei for {111} grains. Further, the pearlite-like carbide is apt to be a starting point of cracks, and there is another aspect that mechanical properties are impaired. That is, according to the method of the present invention, not only the low temperature transformation phase does not exist but also the carbide is finely dispersed in the grains, so that the deep drawing property and mechanical property of the product sheet are improved. Further, as described above, the size of α grains is also small, so that the ridging characteristic is also good.

Here, the cooling temperature range is set to 1200 so that the γ phase does not precipitate.
The reason for limiting the temperature from ℃ to 1000 ℃ is because it is in the γ phase precipitation temperature range of ordinary ferritic stainless steel. Also, the cooling rate was limited to 30 ° C / sec or more in order to suppress the precipitation of the γ phase.
The phase amount can be reduced to 1/2 or less of the total γ phase amount, and can be reduced to 1/4 or less at 50 ° C / sec or more. It is needless to say that the amount of precipitated γ phase that can be tolerated in the present invention is about 10% or less in volume ratio with respect to the mother phase, and that the ferritic stainless steel having a high γ potential needs to have a high cooling rate. The upper limit of the cooling rate is not particularly limited, but it can be controlled practically to 200
Up to about ° C / sec.

The reason for limiting the winding temperature of the thin slab to 1000 ° C or lower and 700 ° C or higher is that the γ phase may precipitate at higher temperatures, and at lower temperatures the effective diffusion distance of atoms is small and forms carbides. Because you can't. The method of cooling the wound thin cast coil is not particularly limited, but usually 100
It is assumed that the cooling rate is not higher than ° C / hour.
Needless to say, slow cooling or heat retention is effective.

The present invention is intended to omit the hot rolling step and the hot rolled sheet annealing step at once, but if the hot rolling step is simply omitted, the slab winding in the present invention is performed. It goes without saying that it is sufficient to anneal the hot rolled sheet under the removal conditions.

Next, the reason why rolling is performed with a rolling reduction of 10% or more between solidification and winding will be described.

In the case of the thin slab of the present invention, voids often exist inside the slab as compared with the conventional hot-rolled sheet. Such voids may deteriorate the cold ductility of the slab and the mechanical properties of the product sheet. Therefore, if the slab has a sensible heat after solidification and is rolled by applying a reduction of 10% or more to eliminate the voids,
The above problems are solved. The reason for limiting the rolling reduction to 10% or more is that the effect of eliminating voids is poor at a rolling reduction of less than this. Although the upper limit of the rolling reduction is not particularly limited, the rolling reduction per pass is practically up to 70%. Although the number of passes is not particularly limited, it is desirable that the number of passes is about 1 to 3 from the viewpoint of facility cost.

Next, the reasons for limiting the components of the starting material of the present invention will be described.

The reason why the Cr content is 8% or more is that the corrosion resistance is inferior when the Cr content is less than this. Corrosion resistance improves as the amount of Cr added increases, but if it exceeds 30%, the effect is small and cold rolling property deteriorates.
Considering the economical efficiency, the Cr content above this is not preferable.
The upper limit was 30%.

The C content is set to 0.001% or more because it is difficult to melt a starting material having a C content less than 0.001% by the usual method.
% And above. The higher the amount of C added, the better the ridging property, but if it is added in excess of 0.5%, the cold ductility and r value deteriorate, so the upper limit was made 0.5%.

The components of the starting material in the present invention are within the scope of the present invention even if any element is contained as long as Cr is in the range of 8 to 30% and becomes α + (carbide) at room temperature. However, even in the case of a component composition such that it becomes an α single phase in all temperature ranges, it is also included in the scope of the present invention, but as a component achieving the main object of the present invention, α, γ at high temperature
It goes without saying that the component system has two phases.

Therefore, it goes without saying that the effect of the present invention is not impaired even if a nitride forming element such as Al or Ti is added in order to improve the mechanical properties and deep drawing properties of the product sheet.

The present invention will be described in detail below with reference to examples.

(Example) Example 1 A ferritic stainless steel having the components shown in Table 1 was cast into a thin-walled slab having a thickness of 2 mm by the twin roll method, and immediately water-cooled to 800
It was wound up at ℃. Cooling rate from solidification temperature to 1200 ℃ is approx.
70 ℃ / sec, cooling rate from 1200 ℃ to 1000 ℃ is about 60 ℃ /
The cooling rate after winding in seconds is about 50 ° C./hour. For comparison, a coil which was air-cooled after casting and wound at 800 ° C. was also manufactured.
The cooling rate during air cooling is 20 to 30 ° C / sec.

When the slabs produced as described above were cold-rolled to a thickness of 0.4 mm, those that were water-cooled after casting according to the present invention and wound up at 750 ° C. showed good cold ductility, but those that were air-cooled were The cold-rolled sheet had cracks in the ears, and the variation in sheet thickness was large.

(Example 2) Ferritic stainless steels A, B, and C having the chemical components shown in Table 2 were melted and made into thin steel plates by the manufacturing process shown in Table 3. Process I is a cold rolling machine with a roll diameter of 150 mm after water-cooling a slab cast with a twin roll casting machine to a thickness of 1 to 2 mm from 1200 ℃ to 1000 ℃, winding at 800 ℃ to make a coil, pickling Was cold-rolled to a thickness of 0.4 mm and annealed at 875 ° C for 60 seconds to obtain a product plate. In Table 3, cooling I means cooling from the solidification temperature to 1200 ° C, and cooling II means 1200 ° C to 10 ° C.
Means cooling to 00 ° C. In Process II, the test steel was cast into a mold having a slab thickness of 4 mm, and then both surfaces were planed to a thickness of 2.0 mm. Process III was cast into a mold having a slab thickness of 20 mm, and thereafter, a thin steel plate was prepared in the same manner as Process II.

Process IV is to cast the sample steel C into a mold with a slab thickness of 20 mm.
After making a 20 mm thick slab, hot rolling was immediately performed for 6 passes 3.
A hot rolled sheet having a thickness of 0 mm was used. Hot rolling start temperature at this time is 1100 ℃
The end temperature was 921 ° C. This hot rolled sheet was annealed at 840 ° C. for 4 hours, and thereafter, a thin steel sheet was obtained in the same manner as in Process I.

Process V is 250 mm in the standard continuous casting (CC) of sample steel C
A thick slab, heated to 1200 ° C, is subjected to 6-pass rough hot rolling and 6-pass finish hot rolling with an ordinary hot strip mill.
The hot rolled steel sheet rolled up at 0 ° C was annealed at 840 ° C for 4 hours, and thereafter, a thin steel sheet was obtained in the same manner as in Process I. Process VI
Was made into a slab of 250 mm in thickness by using standard steel C, heated to 1200 ° C, subjected to 6-pass rough hot rolling and 6-pass finish hot rolling with a hot strip mill, and wound at 580 ° C. Hot rolled steel sheet,
It was pickled, then directly cold rolled and annealed to obtain a thin steel sheet.

Table 4 shows the ridging characteristics of the product sheet manufactured as described above, together with the thickness of each cast piece. For steel types A, B and C, processes I, II and II
Fig. 2 shows the relationship between the ridging characteristics of the product plate manufactured in I and the thickness of the slab. From Table 4 and FIG. 2, it can be seen that the ridging characteristics of the ferritic stainless steel sheet produced according to the method of the present invention are better than those produced by the comparative method or the conventional method.

Example 3 A ferritic stainless steel sheet having the chemical composition shown in Table 5 was cast with a twin roll casting machine to a size of 2 mm, and then thin cast coils were formed under the cold rolling and winding conditions shown in Table 6 and then pickled. hand
Cold rolled to a thickness of 0.4 mm with a roll diameter of 150 mm, and rolled at 875 ° C for 6
It was annealed for 0 seconds to obtain a product plate. Table 7 shows the cold ductility and material properties of the product sheet at that time. Table 7 also shows the material properties of the product plate when the ferritic stainless steel having the components shown in Table 5 was manufactured according to the conventional method (Process V of Example 2).

As shown in Table 7, the material manufactured according to the method of the present invention has substantially the same material characteristics as the conventional method, although the hot rolling step and the hot rolled sheet annealing step are omitted. Has a slightly low total elongation value, but when the slab is subjected to the rolling of the present invention, the total elongation value is considerably improved.

In the comparative method in which air was cooled between 1200 ° C and 1000 ° C, the γ phase was precipitated during cooling, so the pearlite phase (α phase and the layered phase of (Fe, Cr) ₂₃ C ₆ ) in the slab Because there are a lot of
The cold rolling property is poor, such as the occurrence of ear cracks, and the total elongation value and value are low. The comparative method of slowly cooling from the solidification temperature to 1200 ° C
The columnar structure is coarsened and the ridging property is deteriorated. Further, in the comparative method in which the winding temperature is 600 ° C., since carbides cannot be sufficiently precipitated during winding, cold rolling properties such as cold rolling cracks are remarkably poor and yield stress is high and deep drawing properties are also poor.

(Effects of the Invention) As described in detail above, if a ferritic stainless steel sheet is manufactured by omitting the hot rolling step and hot rolling sheet annealing step according to the present invention, the conventional hot rolling step and hot rolling sheet annealing step are performed. It is possible to obtain a ferritic stainless steel sheet having good workability equivalent to that produced by the above method, and to greatly reduce the production cost, which is a great industrial benefit.

[Brief description of drawings]

Fig. 1 shows casting of ferritic stainless steel containing 16 wt% of Cr and 0.02 wt% of C when cast by a single roll casting machine, twin roll casting machine, flat mold, CC (continuous casting machine). Diagram showing the relationship between the piece thickness and the width of the columnar crystals of the cast structure,
Fig. 2 is a photographic diagram showing the metal casting structure when ferritic stainless steel containing 16 wt% Cr and 0.02 wt% C was cast to a thickness of 1 mm by a twin roll casting machine, and Fig. 3 shows Cr.
Fig. 4 is a diagram showing the metal casting structure when a ferritic stainless steel containing 16% by weight and 0.02% by weight C was cast in a flat mold with a slab thickness of 4 mm. Fig. 4 shows the ridging height and the casting of the product sheet. It is a figure which shows the relationship with one side thickness.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Michio Endo 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Inside Nippon Steel Co., Ltd. Technical Research Institute (72) Tetsuya Sugai 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Address Nippon Steel & Steel Co., Ltd. Technical Research Institute No. 1 (56) Reference JP-A-57-25203 (JP, A) JP-A-62-176649 (JP, A)

Claims (2)

[Claims] 1. A ferritic stainless steel containing Cr: 8 to 30% and C: 0.001 to 0.5% in weight% is continuously cast to have a thickness.
Obtaining a slab of 3 mm or less, cooling the slab from the solidification temperature to 1200 ° C at a cooling rate of air cooling or higher, and then 1200 ° C to 1000 ° C
Up to 1000 ℃ after cooling up to 30 ℃ / sec or more
A method for producing a ferritic stainless steel sheet having good workability, which is characterized by forming a thin cast coil by winding in a temperature range of 700 ° C or higher, and then cold rolling and annealing to obtain a thin steel sheet. 2. A ferritic stainless steel containing Cr: 8 to 30% and C: 0.001 to 0.5% in weight% is continuously cast to have a thickness.
Obtaining a slab of 3 mm or less, cooling the slab from the solidification temperature to 1200 ° C at a cooling rate of air cooling or higher, and then 1200 ° C to 1000 ° C
Cooling at a cooling rate of 30 ℃ / sec or more, and after the cooling process from 1200 ℃ to 1000 ℃ or after this cooling process, the rolling reduction of 10% or more, and then 1000 ℃ A method for producing a ferritic stainless steel sheet having good workability, which is characterized by forming a thin cast coil by winding in a temperature range of 700 ° C or higher, and then cold rolling and annealing to obtain a thin steel sheet.